Laser welding is used in various fields because an energy density of the laser beam of the heat source is high and thus low distortion, high speed, and high precision welded joint can be obtained. In the automotive field, there are many products that are welded by stacking or butting together a plurality of materials to be welded to a steel material such as stainless steel or carbon steel, or a metallic material such as an aluminum alloy or a nickel alloy. For example, in the manufacturing of a vehicle body, a fuel pump, an injector (fuel injection valve), and a welding process using a pulse wave or a continuous wave laser light is used.
In addition, in order to improve structure and performance of the surface or a portion of the metallic material, an additive material (wire or powder) having a different component from that of the metallic material is used, and surface reforming for improving the local structure and performance of the metallic material by laser beam irradiation is also applied to a wide range of products.
For example, in order to improve the responsiveness of fuel injection valves used in the automobile field, a local non-magnetic or low-magnetization treatment is performed on the pipe-shaped magnetic material.
However, for a thin pipe, a drop of a reformed portion into an inside surface of the pipe is generated by the laser irradiation and a shape adjustment step is necessary after the low-magnetization treatment.
The reason for the generation of the drop described above is that when irradiating a predetermined low-magnetization treatment area with the laser beam, since a width of a treatment area required from the performance of the fuel injection valve can be obtained, it is necessary to set the laser beam spot diameter to increase or to set the heat input amount of the laser beam to increase. In this case, there is a case where melting down is generated or a case where the thin pipe is greatly deformed, by an increase in the amount of heat input to the thin pipe.
In order to solve the problem of the drop described above, PTL 1 describes a method of performing laser irradiation while supplying a non-oxidizing shielding gas inside the pipe.